nrf51822 ADC pin internal resistance

Hi. I am looking for the internal resistance of the ADC pin of my nrf51822. I have looked at the Product Specifications (as well as the Reference Manual and other documents just in case...) but didn't manage to find it :( . I saw typical values for other boards (Pi, STM32, etc.) but I need the exact value in the case of my Nordic nrf51822 board since I need to compute a precise value based on this resistance. Does anyone know where (or how) I can find (or determine) this value? Thanks in advance.